The present invention relates generally to microphones for voice and music, and more particularly to an improved pressure microphone having superior vibration and noise rejection. Heretofore, pressure microphones have generally comprised a single, approximately planar vibratile element or diaphragm open to ambient sound or pressure variations on one side and essentially sealed from these variations on the other. The position of the diaphragm at any instant is related to the difference between ambient pressure on the open side and the pressure of the sealed volume of air on the opposite side. Various transduction means are utilized to convert these variations in position to electrical signals, which signals ideally become replicas of the ambient pressure variations. Added to these signals, however, are the undesired diaphragm motions, caused by mechanical vibration transmitted through the microphone structure, and by noise contributed by electrical resistance of the transducing means or succeeding stages of amplification. Prior art means for increasing the sensitivity, and thus the signal-to-noise ratio, of microphones have generally involved increasing the size of the vibratile element or improving the efficiency of the transducing means whereby diaphragm motion is converted into electrical signals. In the latter case, any increase in transduction efficiency results in a like increase in efficiency for structure-borne vibration and shock-induced signals.
One means for improving overall efficiency of a microphone, as disclosed in U.S. Pat. No. 3,980,838 to Yakushiji, et al., involves electrostatic diaphragms disposed about a perforate common electrode. The device is used as a loud speaker approximating a plate the thickness of which grows and shrinks to follow the waveform being reproduced. The technique of dual diaphragms about a common perforate electrode is disclosed as early as 1935 in U.S. Pat. No. 2,179,361 to von Braunwahl, et al., the object being to provide a directional response.
An example of a microphone of the prior art is the aircraft radio noise cancelling microphone, which has a single transducer and a bidirectional acoustic channel driving the transducer from opposite directions, whereby bidirectional balanced pressures, such as ambient noise, are substantially cancelled. Acoustically unbalanced voice pressure enables substantially noise-free microphone output and improved dynamic range.